This invention relates to a pulp lifter for installation in a grinding mill.
A conventional pulp lifter for a grate discharge mill comprises a plurality of chambers radially arranged to rotate against the downstream side of a vertical or sloped grate. Each pulp lifter chamber is defined between a trailing edge wall and a leading edge wall, relative to the direction of rotation of the mill. In the conventional pulp lifter, the trailing edge wall and leading edge wall are radial, and the trailing edge wall of a leading pulp lifter chamber is the leading edge wall of the next following pulp lifter chamber. The pulp lifter chambers are open towards the axis of the mill.
A mill charge of mineral or mixture of mineral and any grinding media on the upstream side of the grate tumbles as the mill rotates. Water is fed to the mill and as the mineral is comminuted by the tumbling action, the fine particles and the water form a slurry in the interstices of the mineral. Some of the slurry passes through the apertures in the grate. During a portion of each rotation of the mill, each pulp lifter chamber in turn passes against the mill charge on the upstream side of the grate and slurry passes through the grate to a collecting region of the pulp lifter chamber.
As the mill rotates, the material in the pulp lifter chamber is lifted upward. The orientation of the pulp lifter chamber changes until ultimately the chamber is open downwards and material may fall downward from the chamber onto a discharge cone, which directs the material towards a discharge opening of the mill.
Developments of the conventional pulp lifter are described in U.S. Pat. No. 7,566,017 issued Jul. 28, 2009 and International Publication No. WO 98/01226, the entire disclosure of each of which is hereby incorporated by reference herein for all purposes. The pulp lifter disclosed in U.S. Pat. No. 7,566,017 is partially modular, in that each pulp lifter chamber is formed by a separate pulp lifter module, and the separate modules are assembled in a support structure. Moreover, the grate is integrated into the pulp lifter modules.
The material that enters a pulp lifter chamber through the grate has two principal fractions, namely a slurry fraction, composed of water and particles that are smaller than about few millimeters, and a pebble fraction, composed principally of stones that are larger than about few centimeters. The discharge position of the slurry depends on the mill rotational speed and the effective mill diameter. When the mill is viewed as rotating in the counterclockwise direction, the slurry fraction in a pulp lifter chamber starts flowing toward the discharge cone when the pulp lifter chamber is at about the 2:00 o'clock position and is discharged almost completely by the time that the pulp lifter chamber attains the 10:30 to 11:00 o'clock position. The pebble fraction on the other hand moves much less easily and does not start to fall toward the discharge cone of the pulp lifter until the pulp lifter chamber reaches about the 1:00 o'clock position, depending on the mill speed. For a short interval of rotation about the 12:00 o'clock position, the pebbles fall freely but from about 11:00 o'clock to the 10:00 o'clock position they strike the leading edge wall of the pebble lifter chamber and slide down the leading edge wall. After 10:00 o'clock, the sliding movement of the pebble fraction slows down and in any event any pebbles that fall from the pulp lifter chamber might not be discharged by the discharge cone but fall into another chamber of the pulp lifter. Thus, a large proportion of the pebble fraction is not discharged but remains in the pulp lifter over several rotations. This operation of the conventional pulp lifter is illustrated in FIG. 1.
The recycling pebbles form a dead load behind the grate, which reduces the volumetric capacity of the pulp lifters by partially occupying the effective volume of the pulp lifters and increases the mass of the mill. In addition, the recycling pebbles may block the grate openings, and the presence of a quantity of pebbles in the pulp lifter reduces the flow gradient through the grate, and may cause a slurry pool to be formed in the mill. It is therefore desirable to reduce the proportion of the pebble fraction that remains in the pebble lifter over multiple rotations of the mill.
The object of the present invention is to eliminate drawbacks of the prior art and to achieve a more effective apparatus for discharging material from a mill, which is used for grinding or comminution, even at the higher rotating speeds of the mill.